The invention relates to an electrode arrangement for plasma-aided coating.
Such an electrode arrangement is, for instance, employed to deposit one or more layers of a material on a substrate surface so that the substrate offers certain physical properties over its surface. Applications are, for example, the coating of a glass sheet with an optically active layer so that the glass sheet will possess desired reflecting or, respectively, transmission properties, or, as another example, the coating of a plastic foil with a layer of a material so that the plastic foil will be active as a diffusion barrier.
The patent publication PCT/EP 99/00768 discloses an electrode arrangement for the coating of a substrate with a layer of a material, which comprises two material components, the second material component being comprised as a doping agent in the material layer to a lesser extent than the first material component. The first material component is provided by evaporation from a crucible, whereas the second material component is provided from the surface of an electrode, which is placed in a circuit as a cathode in relation to the crucible. Between the cathode and the crucible playing the role of an anode, an arc discharge is struck using a suitably selected potential difference between the anode and the cathode and a gas present between the anode and the cathode. Owing to such arc discharge particles of the second material component become detached from the cathode surface, mix and react with the particles of the first material component evaporated from the anode with the result that a material layer is deposited on a substrate surface arranged in the vicinity of the cathode and of the anode, such deposited material layer containing both the two material components. The proportions of the two material components in the deposited material layer may be challenged because the degree of evaporation from the crucible and the intensity of the arc discharge may be varied to a certain degree independently of each other.
The intensity and efficiency of the arc discharge is, in this case, influenced by the surface properties of the cathode. In the case of the conventional electrode arrangement, there is a problem to the extent that particles evaporated from the anode of the first material component or, respectively, reaction products thereof deposit on the surface of the cathode constituting the second material component. This means that the surface properties of the cathode change during the operation of the electrode arrangement, something leading to a change with time in the intensity and the efficiency of the arc discharge and accordingly to an impairment regarding deposition of material layers with a predetermined composition on the substrate. More particularly, it may be the case that deposits of the first material component on the cathode may result in the arc discharge being extinguished and further operation of the electrode arrangement with the contaminated cathode being impossible. Then replacement of the cathode is essential, and this will mean a disadvantage regarding the efficiency of production of the material layer.